Tomato plants colonised with the arbuscular mycorrhizal (AM) fungus Glomus fasciculatum show systemic induced resistance to the foliar pathogen Alternaria alternata, as observed in interactions of other AM-colonised plants with a range of pathogens. The role of jasmonic (JA) and salicylic (SA) acid in expression of this mycorrhiza-induced resistance (MIR) against A. alternata was studied by measuring: (i) activity of enzymes reported to be involved in their biosynthesis, namely lipoxygenase (LOX) and phenylammonia lyase (PAL); and (ii) levels of methyl jasmonate (MeJA) and SA. Transcript abundance of some defence genes associated with JA and SA response pathways were also studied. Both LOX and PAL activity increased twofold in response to pathogen application to control plants. AM-colonised plants had three-fold higher LOX activity compared to control plants, but unlike controls, this did not increase further in response to pathogen application. Higher LOX activity in AM-colonised plants correlated with four-fold higher MeJA in leaves of AM-colonised plants compared to controls. Treatment of plants with the JA biosynthesis inhibitor salicylhydroxamic acid (SHAM) led to 50% lower MeJA in both control and AM-colonised plants and correlated with increased susceptibility to A. alternata, suggesting a causal role for JA in expression of MIR against the pathogen. Genes involved in JA biosynthesis (OPR3) and response (COI1) showed six- and 42-fold higher expression, respectively, in leaves of AM-colonised plants compared to controls. AM-colonised plants also showed increased expression of the SA response gene PR1 and that of the wound-inducible polypeptide prosystemin. Our results suggest that the systemic increase in JA in response to AM colonisation plays a key role in expression of MIR against A. alternata.

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Arbscular mycorrhizal (AM) symbiosis is widely distributed in the plant kingdom, unlike rhizobial symbiosis that is observed only in four orders of the eurosid dicots.1 AM symbiosis involves provision of mineral nutrients to the plant by the fungus, which in turn derives carbon compounds from the plant.2 Other benefits of the symbiosis to plants include improved water relations and tolerance to some plant diseases.3 The fungal hyphae penetrate the root epidermis, spread inter-cellularly in the cortex and form arbuscules in the root cells of the inner cortex. Arbuscules are highly branched hyphal structures that are separated from the plant cytoplasm by a perifungal membrane. 4 They are the main sites where nutrient exchange between the symbiotic partners occurs and their formation signifies the establishment of functional symbiosis.Molecular events occurring during AM symbiosis have been studied in legumes, which also show an elaborate signaling pathway for establishing nodulation symbiosis.5 Two genes, NFR1 and NFR5 encoding receptor-like serine/threonine kinases with LysM domains, are involved in nodulation (Nod) factor perception in Lotus japonicus.6 Several downstream components of the Nod factor signaling cascade include the leucine-rich-repeat receptor kinase SYMRK, which is known to be involved in AM symbiosis besides nodulation symbiosis and is thought to act near the junction of fungal and rhizobial signaling cascades. 7Activation of SYMRK causes a transient increase in intracellular calcium levels. Downstream components of this signaling pathway include a calcium/calmodulin dependent protein kinase (CCAMK ) and a protein CYCLOPS, whose function is notComparison of the expression of 13 genes involved in arbuscular mycorrhizal (am) symbiosis was performed in a wild type tomato (Solanum lycopersicum cv. 76r) and its reduced mycorrhizal colonization mutant rmc in response to colonization with Glomus fasiculatum. Four defense-related genes were induced to a similar extent in the mutant and wild type am colonized plants, indicating a systemic response to am colonization. Genes related to nutrient exchange between the symbiont partners showed higher expression in the am roots of wild type plants than the mutant plants, which correlated with their arbuscular frequency. a symbiosis receptor kinase that is involved in both nodulation and am symbiosis was not expressed in the rmc mutant. the fact that some colonization was observed in rmc was suggestive of the existence of an alternate colonization signaling pathway for am symbiosis in this mutant.

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